While there are a large number of vehicles such as earthmoving tractors and military tanks that have utilized multi-speed cross drive transmissions, the transmissions have incorporated an excessive number of gears and associated elements, and have generally been overly complex and costly in construction. In some cases multiple speeds for the driving ground-engaging members have been achieved by merely duplicating a plurality of planetary sets and operating brakes and clutches on the left and right sides, with such brakes and clutches controlling one or more reaction members independently of the operation on the opposite side of the vehicle.
Another deficiency with prior cross drive mechanisms is that the rotating members thereof have been arranged on a plurality of transverse axes so that the construction was not only costly, but also required an excessively large housing for containment of the components.
Still another deficiency is that such cross drive transmissions have not simultaneously provided a true differential function wherein the driving torques at the opposite sides of the vehicle are continuously equalized. For example, differential steering mechanisms have heretofor been proposed which force one output member to increase in speed while simultaneously forcing the opposite output member to decrease in speed by acting on one element of a pair of oppositely disposed planetary sets. This is achieved by a cross shaft or bevel gear drive connection between such elements and separate from the input drive that interferes with the true differential function and causes unequal torques to be imposed upon the output members. Moreover, hydrostatic drive units have been employed for steering purposes in many of the earlier mechanisms, but the mixture of hydrostatic and mechanical drive unduly complicates the construction and requires an excessive amount of space.
Thus, what is needed is a compact multi-speed differential having a true differential action and a minimum number of components which can preferably be arranged along one transverse axis in a concentric manner. Preferably too, the multi-speed differential should be simple and economical to manufacture, and should incorporate a built-in reduction or speed step down capability so that it can be relatively closely coupled to a driving engine without the need for an additional reduction gear train.
The present invention is directed to overcoming one or more of the problems as set forth above.